The present invention relates to a copier, laser printer, facsimile apparatus or similar image forming apparatus and, more particularly, to an image forming apparatus of the type collecting and transferring a developer removed after image formation.
An electrophotographic copying system belongs to a family of image forming systems and implemented as a copier, printer or facsimile apparatus by way of example. In the electrophotographic copying system, an image carrier in the form of a photoconductive element is uniformly charged and then exposed or scanned to form a latent image electrostatically thereon. The latent image is developed by toner included in a two-ingredient type developer, i.e., toner and carrier mixture or a single-ingredient type developer. The resulting toner image is transferred to a sheet or similar recording medium to produce a copy.
The developer is sequentially consumed due to repeated development and must be replenished when consumed by more than a preselected amount. As for the two-ingredient type developer, for example, the amount of toner decreases due t o aging with the result that the toner content of the developer existing in a developing device becomes too low to maintain desired image density. It has been customary with the above image forming apparatus to replenish fresh toner to the developing device when the toner content falls below a preselected value, thereby maintaining the toner content stable. For this purpose, the image forming apparatus includes a toner replenishing device.
Further, the image forming apparatus includes a cleaning device for removing the toner left on the photoconductive element after the transfer of the toner image to the sheet. An exclusive cleaning device is also associated with, e.g., a conveying device for conveying the sheet with the toner image to a fixing device, because the toner remains on the conveying device also. The toner removed by such cleaning devices sequentially increase due to repeated image formation and fill them up in due course of time. To insure desirable cleaning, the toner collected in the cleaning devices must be suitably discharged. For this purpose, a toner storing device for receiving the collected toner is used.
For the replenishment of toner from the toner replenishing device to the developing device and for the transfer of the collected toner from the cleaning devices to the toner storing device, the devices are communicated by pipes each accommodating a coil screw therein. The coil screw is sometimes replaced with a paddle, bucket or the like. Further, the toner may be dropped into the toner storing device by gravity.
The current trend in the imaging art is toward the recycling of the collected toner for promoting the effective use of limited resources. For example, a mechanism capable of transferring the toner collected in the cleaning device to the developing device has been proposed. The mechanism may include a pipe communicating the toner outlet of the cleaning device and a toner storing section provided independently of the cleaning device or the developing device. A conveyor screw is disposed in the pipe for transferring the toner from the cleaning device to the toner storing section or the developing device. Again, the toner may be dropped into the toner storing section by gravity.
The conventional toner transferring systems described above have some problems yet to be solved, as follows. The coil screw, for example, must extend to the vicinity of the developing device or the toner storing device. To insure the rotation of the coil screw, a linear path, path with a great radius of curvature or similar severely restricted path must be provided. Therefore, even the coil screw scheme limits t h e layout of the apparatus, not to speak of the gravity scheme. This limits the design freedom of the apparatus and sometimes complicates the construction and increases the size of the apparatus.
When the force for moving the toner is excessive, a heavy mechanical stress is apt to act on the toner. Particularly, when the force compressing toner particles against each other is great, the resulting heat causes the toner to melt and cohere (so-called blocking) and deteriorates the conveyance of the toner. In addition, the above force is apt to crush the toner and degrades the charging characteristic of the toner, rendering the toner inadequate for development. As for the pipe and screw scheme, the distance of toner transfer effected by the screw must be as small as possible. An increase in the distance would increase the torque for rotating the screw and would thereby aggravate t h e mechanical stress.
To meet the demand for the downsizing of the image forming apparatus, it is necessary to scale down the toner transferring means and toner storing section. However, the conventional mechanical toner transferring means cannot be scaled down beyond a certain limit in relation to the required toner conveying force. Assume that the toner replenishing section is implemented as the toner storing section. Then, if the toner replenishing section is scaled down, then it runs out of toner frequently and must be dismounted and replaced frequently. To replace the toner replenishing section, the operation of the apparatus must be interrupted and then resumed later, wasting time. Even when the toner storing section is used to store the collected toner, it must be replaced when filled up. Replacing this toner storing section often makes the section provided in addition to the cleaning device meaningless.
Under the above circumstances, the toner storing section needs a certain degree of capacity. As the apparatus becomes bulky and sophisticated, there arise not only a space problem but also a manipulation problem regarding maintenance including the replacement of the toner storing section. Further, the maintenance increases the down time of the apparatus or the frequency of interruption of operation. Because the down time of the apparatus is proportional to the period of time necessary for maintenance to be completed, an increase in the size of the toner storing section directly translates into an increase in time and labor, lowering the operation ratio of the apparatus.